Method for manufacturing homogeneous bodies from semiconductor alloys

ABSTRACT

In a method of manufacturing a homogeneous, fissure-free body from a semiconductor alloy having a negative coefficient of expansion in the region of its melting point and contained in an upright tube having a closed bottom and solidified in the tube after having been in a molten condition therein, the step of rotating the tube about its axis at a speed producing cavitation in the molten alloy at least during solidification of the molten alloy therein. In an apparatus for carrying out the foregoing method, there is provided an elongated tubular container adapted to contain the alloy in molten condition and during solidification thereof, means operatively connected to the container for rotating the latter about its axis at a speed producing cavitation in the molten alloy, and a high-frequency coil surrounding at least part of the container and having an axis coinciding at least substantially with the axis of the tubular container.

United States atent (lesterhelt et a1 BEST AVAILABLE COPY JuEy'ZS, 1972 [54] METHOD FUR MANUFACTURING 3,152,373 10/1964 Einthoven et al.... ..148/1.6 HOMOGENEOUS BODIES FROM 2,747,971 5/1956 ..148/1 6 SEMICQNDUCTOR ALLOYS 1,745,886 2/1930 164/118 X 1,812,172 6/1931 ..164/51 [72] Inventors: Gerhard OeSterheIt, urnberg; Eugen 1,844,701 2/1932 Tama .164/1 18 Szab De Bucs, Erlangen, both of Germany - Primary Exammer--J. Spencer Overholser [73] Assignee: Siemens-Schuckertwerke Aktien- A E v Rising gesellschflft, Berlin and Erlflngen. Atrorney-Arthur E. Wilfond, Herbert L. Lerner and Daniel .1, many Tick [22] Filed: April 27, 1970 21 Appl. No.: 29,771 [57] ABSTRACT In a method of manufacturing a homogeneous, fissure-free Related Apphcatlon Data body from a semiconductor alloy having a negative coefficient 63 Continuation f s No, 751 00 J l 25 1968 of expansion in the region of its melting point and contained in abandoned, which i a Continuation f S N an upright tube having a closed bottom and solidified in the 50439 O 24 1965 abandoned tube after having been in a molten condition therein, the step of rotating the tube about its axis at a speed producing cavita- [30] For i A li atio P iorit Data tion in the molten alloy at least during solidification of the molten alloy therein. June 5, 1965 Germany ..597497 In an apparatus for carrying out the foregoing method, there is [52] US Cl ..164/51, 252/623, 148/15, provided an elongated tubular container adapted to contain 126/3435, 13/27, 219/10.75, 164/251, 164/118, the alloy in molten condition and during solidification thereof, 29/589, 29/569, 164/96, 164/136, 164/286 means operatively connected to the container for rotating the [51] Int. Cl ..B22d 27/02 latter about its axis at a speed producing cavitation in the mo]- [58] Field of Search ..148/l.6; 164/60, 51, 1 18 ten alloy, and a high-frequency coil surrounding at least part of the container and having an axis coinciding at least substan- [56] References Cited tially with the axis of the tubular container.

UNITED STATES PATENTS 1 Claim, 2 Drawing Figures 3,367,394 2/1968 Roder et al. ..164/46 lllf/AAAL 111177111 Patented July 25, 1972 Ill llllllllllllllllnl METHOD FOR MANUFACTURING IIOMOGENEOUS BODIES FROM SEMICONDUCTOR ALLOYS This case is a continuation of Ser. No. 751,006 filed July 25, 1968, abandoned, which is a continuation of Serial No. 504,896 filed Oct. 24, I965, abandoned.

Our invention relates to a method and apparatus for manufacturing a homogeneous body from a molten semiconductor alloy situated within an upright tubular container which has a closed bottom.

In this type of method and apparatus particularly great difficulties are encountered in the case where the coefficient of expansion of the semiconductor alloy is negative in the region of its melting point. Such a negative coefficient of expansion is encountered, for example, in the case of an alloy of germanium and silicon. In order to manufacture such alloys it has been customary up to the present time to use the horizontal zonelevelling method with argon rinsing as the most suitable method of manufacture of such bodies, even though this method necessarily carries with it the very serious disadvantage of partial separation of the alloy components from each other. The term Zone-levelling method" is applied to a zone-melting process where a relatively small axially localized melting zone is alternately shifted first in one direction and then the opposite direction along a semiconductor ingot. From a phase diagram of germanium-silicon it is apparent that upon simple solidification of the molten alloy the components thereof become separated. This separation effect can be reduced by diffusion methods at the boundary between the solid and liquid material. Since, however, the coefficient of diffusion of silicon in a crystalline germanium-silicon mixture is very small, it is necessary, in order to achieve any appreciable reduction in the separation effect, to maintain the speed of movement of the melting zone sufficiently small. It is possible to achieve a good homogeneity under these conditions with i the known method of the type referred to above only when the speed of movement of the melting zone is in the region of 3-5 mm.per hour and only where the melting zone is passed back and forth along the ingot so as to make 3-5 passes therealong. Under these conditions, the time required to manufacture a 7 body of, for example, a length of 70 mm. is on the order of 70 hours with the known methods. However, even under these conditions fissures in the semiconductor body are encountered, so that a considerable part of the semiconductor body which is manufactured in this way cannot be used.

It is a primary object of our invention to provide a method of the above general type which, however, makes it possible to manufacture the semiconductor body in a much shorter time than was heretofore possible.

lt'is furthermore an object of our invention to provide a method and apparatus which not only require less time than conventional methods and apparatus for the manufacture of semiconductor bodies under the above conditions, but which in addition will produce bodies which are free of fissures, so that the method and apparatus of our invention is highly practical.

Our invention is of particular significance in the case where a doped semiconductor material is manufactured with the dopant being an easily volatilized material. Where the dopant is readily volatilized, in the conventional zone-levelling method a relatively large proportion of the dopant will escape from the molten material because of the extremely long manufacturing timeand because of the relatively large exposed surface of the molten material. Therefore, with this conventional method and apparatus it is hardly possible to achieve an accurately doped material and a homogeneous distribution of the dopant.

Thus, it is a further object of our invention to provide a method and apparatus which make it possible to manufacture materials of the above type under conditions where the loss of easily volatilized dopants is extremely low because of the extremely short time required for the manufacture of the semiconductor body with the method and apparatus of our invention and also because of the fact that the molten material with our invention has an extremely small exposed surface as compared to the total amount of material which is in a molten condition.

A further object of our invention is to provide a semiconductor body which is in cylindrical form, so as to have the most favorable configuration for further treatment of the body.

In addition, it is an object of our invention to provide a body which requires only small head and tail ends of the ingot to be separated and to be considered as waste.

Furthermore, it is an object of our invention to provide a method and apparatus of the above type which requires such a small time for the manufacture of the semiconductor body that rapid testing of a series of manufactured bodies can be brought about so as to determine according to the measurements of the electrical properties whether or not any changes in the proportions of the components weighed into the initial charge are required.

In order to achieve a p-type germanium-silicon alloy, the base material is preferably doped with elements from the third main group of the periodic system, such as, for example, boron, gallium, or indium. An n-type germanium-silicon alloy is preferably achieved with doping with elements from the fifth main group of the periodic system, such as, for example, phosphorous, arsenic, or antimony. The individual centrifugal castings are initially prepared by weighing in the corresponding amounts of the initial ingredients, and because of the relatively low melting point of the dopant for n-type materials, these dopants are introduced into the bottomof the quartz tubular container before the other ingredients. In this way when the entire charge is rendered molten, the escape of the dopant by vaporizing out of the molten charge is avoided.

A high-frequency coil is used for melting the semiconductor alloy. Cooling then follows either by reducing the highfrequency energy or by removing the tubular container with the molten charge therein from the high-frequency field. In the latter case it is desirable, in order to maintain as continuous a transition as possible, to use a high-frequency coil which is funnel-shaped, which is to say a coil which has a cylindrical portion and afrustoconical portion adjoining the cylindrical portion. With this latter type of coil the tubular container for the molten charge is axially moved toward the frustoconical part of the coil where the convolutions thereof become gradually of greater diameter.

In the event that the semiconductor body is required for thermoelectric purposes, then the ends of the body must be provided with electrical contacts. The mounting of a contact at one end, for example in the form of a tungsten contact plate, can be brought about very simply by introducing the tungsten contact plate into the tubular container for the charge before the charge itself is introduced. In this way the semiconductor material will become alloyed directly to the contact plate during the melting and subsequent solidification of the charge.

Our invention is illustrated by way of example in the accompanying drawings in which:

FIG. 1 schematically illustrates one possible apparatus of our invention for practicing the method of our invention; and

FIG. 2 is a fragmentary illustration of a variation in the highfrequency induction coil of FIG. I.

In order to manufacture a homogeneous semiconductor body from a doped germanium-silicon alloy, the required amounts of germanium, silicon and dopant are weighed out, and then introduced into a quartz tube which forms the tubular container for the charge. The materials which volatilize more readily are first introduced into the tube in advance of the less easily volatilized materials. Then the quartz tube is centrally gripped in a centrifuging device and is rinsed with argon gas. Under continued rinsing the charge is heated in the than its melting point, the entire molten charge is centrifuged for approximately 2 minutes. The required speed of rotation is determined in accordance with the diameter of the tube which contains the charge, and this latter diameter is of course determined by the size of the manufactured body which will be achieved, and in addition the specific weight of the materials will have an influence on the required speed of rotation. In the above example of a doped germanium-silicon alloy, for a charge diameter of 9 mm. a speed of rotation of I,500-2,000 revolutions per minute was chosen. After turning off the highfrequency field the centrifuging was continued until the charge solidified. The body which has thus been molten and then solidified while being simultaneously rotated together with the tube is removed from the quartz container, and for the purposes of further working of the body the end sections thereof are cutoff.

The apparatus of our invention which is illustrated in FIG. 1 and which is capable of carrying out the method of our invention includes two assemblies one of which serves for centrifuging and the other of which serves for heating the charge. The centrifuging assembly includes the rotary tube 1 which carries a chuck 2 in which the quartz tube 3 can be introduced and centrally clamped. The tube 1 is supported for rotary movement about its axis by means of ball bearings 4 which are mounted in a metal sleeve 13 and the tube 1 is rotated with a drive from a motor 5 whose speed of rotation can be adjusted. In order to control the speed of rotation of the motor 5 a tachometer 6 is provided, and in this way it is possible to measure the speed of rotation of the centrifuging tube 3. Thus, it will be seen that the motor 5 rotates a pulley 14 which by way of a belt 15 rotates asecond pulley which is fixed to the shaft or tube 1, and the pulley fixed to the hollow tube 1 and driven by the belt 15 is of the same diameter as the pulley 14, so that in this way the tachometer 6 will give an accurate measurement of the speed of rotation of the quartz tubular container 3 for the molten charge.

The tubular shaft 1 accommodates in its interior an additional tube 7 through which the argon gas is introduced.

The heating assembly is made up of a high-frequency induc tion coil 8 which is supplied with power from a high-frequency generator. The high-frequency induction coil is so arranged that it completely surrounds the molten charge 9 situated within the quartz tube 3.

It is to be noted that in the example of FIG. 1 the coil 8 is of a cylindrical configuration in that all of its convolutions are of the same diameter. In contrast, in the embodiment of FIG. 2, the heating coil 10, which is a high-frequency coil used instead of the coil 8, has a portion of frustoconical configuration. This coil 10 has anupper end portion 11 made up of a series of coils of the same diameter so that this upper end portion 11 is cylindrical. However, from the upper portion 11 to the bottom end of the coil, the convolutions thereof become of a gradually greater diameter so that the lower portion 12 of the coil 10 is of a frustoconical configuration with the lowermost convolution having the largest diameter. The quartz container 3 is introduced from above slowly through the coil, in the case of FIG. 2, and in this way the charge is brought into a molten condition in the intense high-frequency field of the cylindrical part II of the coil 10. However, as the charge continues to move through the lower portion 12 where the high-frequency field becomes less intense, the charge cools. Thus, with the process of our invention which uses the coil 10 of FIG. 2, it is possible to melt and subsequently solidify charges which are longer than the total length of the coil.

It is possible to use as the initial material for the manufacturing method of our invention the individual components of the desired alloy or an alloy which has already been placed in a molten condition, or in some cases it is possible to use a mixture in comminuted condition. This last case may be resorted to where extremely high standards of homogeneit are required. Moreover, where the charge IS premelte the weighing of large pieces can be avoided.

It is thus apparent that the objects of our invention are achieved at least in part, in the case where the semiconductor alloy has a coefficient of expansion which is negative in the region of its melting point, by rotating the tubular container for the molten charge about its axis at least during solidification of the charge.

With the method and apparatus of our invention it is possible to manufacture homogeneous semiconductor bodies having a diameter of, for example, 6-15 mm. and a length of approximately l5 mm. in a time period of 5 minutes and these bodies achieved with our invention are free of fissures. As a result of the rotation at least during cooling and solidification, there is provided not only an extremely good and thorough mixing of the materials, but in addition a sink (cavity) forms in the molten mass, and the molten, solidifying material expands into this latter sink because of the negative coefficient of expansion.

We claim:

1. In a method of manufacturing a homogeneous, fissurefree body from a semiconductor alloy which has a negative coefficient of expansion in the region of its melting point and which is contained in an upright tube having a closed bottom and solidified in the tube after having been in a molten condition therein, an initial step of introducing components of the semiconductor alloy into the tube with the most easily volatilized components introduced into the tube in advance of the other components, a second step of situating the tube along the axis of a high-frequency coil having a first portion producing a high-frequency fied of greater intensity for heating the contents of the tube to a temperature at which molten semiconductor alloy is formed, and a second portion producing a high-frequency field of lesser intensity wherein the molten alloy is cooled to solidification temperature, the subsequently introduced components tending to block escape of the more easily volatilized components from the tube as the more easily volatilized components are volatilized by the heating of the tube contents, a third step of axially moving the tube along the axis of said coil out of said first portion and into said second portion thereof, and a fourth step of rotating the tube about its axis at least during solidification of the molten alloy at a speed producing a cavity in the molten alloy. 

